The short version
A combi heats water on demand and needs no tanks. A system boiler works with a hot water cylinder and suits homes where several people want hot water at once. Neither is better in the abstract, only better for a particular house.
When a combi makes sense
Combis suit most flats and two to three bedroom houses in St Albans with one bathroom. They free up the airing cupboard and loft, and you never run out of hot water because there is no stored tank to empty. The catch is flow rate. If two showers run at once, pressure drops noticeably.
When a system boiler is the better call
If you have two or more bathrooms, a large family, or low incoming mains pressure, a system boiler with an unvented cylinder is usually the right answer. Everyone can shower at the same time without the temperature swinging.
The thing most quotes ignore
Boiler size matters more than brand. An oversized boiler cycles on and off, wears faster and costs more to run. We calculate the heat requirement from your radiators, insulation and hot water demand rather than fitting whatever is in the van.
